[PATCHSET v3 sched_ext/for-6.15] sched_ext: Enhance built-in idle selection with allowed CPUs

[Andrea Righi <arighi@nvidia.com>]

Many scx schedulers implement their own hard or soft-affinity rules to
support topology characteristics, such as heterogeneous architectures
(e.g., big.LITTLE, P-cores/E-cores), or to categorize tasks based on
specific properties (e.g., running certain tasks only in a subset of CPUs).

Currently, there is no mechanism that allows to use the built-in idle CPU
selection policy to an arbitrary subset of CPUs. As a result, schedulers
often implement their own idle CPU selection policies, which are typically
similar to one another, leading to a lot of code duplication.

To address this, extend the built-in idle CPU selection policy introducing
the concept of allowed CPUs.

With this concept, BPF schedulers can apply the built-in idle CPU selection
policy to a subset of allowed CPUs, allowing them to implement their own
hard/soft-affinity rules while still using the topology optimizations of
the built-in policy, preventing code duplication across different
schedulers.

To implement this introduce a new helper kfunc scx_bpf_select_cpu_and()
that accepts a cpumask of allowed CPUs:

s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu,
			   u64 wake_flags,
			   const struct cpumask *cpus_allowed, u64 flags);

Example usage
=============

s32 BPF_STRUCT_OPS(foo_select_cpu, struct task_struct *p,
		   s32 prev_cpu, u64 wake_flags)
{
	const struct cpumask *cpus = task_allowed_cpus(p) ?: p->cpus_ptr;
	s32 cpu;

	cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, cpus, 0);
	if (cpu >= 0) {
		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
		return cpu;
	}

	return prev_cpu;
}

Results
=======

Load distribution on a 4 sockets / 4 cores per socket system, simulated
using virtme-ng, running a modified version of scx_bpfland that uses the
new helper scx_bpf_select_cpu_and() and 0xff00 as allowed domain:

     $ vng --cpu 16,sockets=4,cores=4,threads=1
     ...
     $ stress-ng -c 16
     ...
     $ htop
     ...
       0[                         0.0%]   8[||||||||||||||||||||||||100.0%]
       1[                         0.0%]   9[||||||||||||||||||||||||100.0%]
       2[                         0.0%]  10[||||||||||||||||||||||||100.0%]
       3[                         0.0%]  11[||||||||||||||||||||||||100.0%]
       4[                         0.0%]  12[||||||||||||||||||||||||100.0%]
       5[                         0.0%]  13[||||||||||||||||||||||||100.0%]
       6[                         0.0%]  14[||||||||||||||||||||||||100.0%]
       7[                         0.0%]  15[||||||||||||||||||||||||100.0%]

With scx_bpf_select_cpu_dfl() tasks would be distributed evenly across all
the available CPUs.

ChangeLog v2 -> v3:
 - incrementally refactor scx_select_cpu_dfl() to accept idle flags and an
   arbitrary allowed cpumask
 - build scx_bpf_select_cpu_and() on top of the existing logic
 - re-arrange scx_select_cpu_dfl() prototype, aligning the first three
   arguments with select_task_rq()
 - do not use "domain" for the allowed cpumask to avoid potential ambiguity
   with sched_domain

ChangeLog v1 -> v2:
  - rename scx_bpf_select_cpu_pref() to scx_bpf_select_cpu_and() and always
    select idle CPUs strictly within the allowed domain
  - rename preferred CPUs -> allowed CPU
  - drop %SCX_PICK_IDLE_IN_PREF (not required anymore)
  - deprecate scx_bpf_select_cpu_dfl() in favor of scx_bpf_select_cpu_and()
    and provide all the required backward compatibility boilerplate

Andrea Righi (8):
      sched_ext: idle: Honor idle flags in the built-in idle selection policy
      sched_ext: idle: Refactor scx_select_cpu_dfl()
      sched_ext: idle: Extend topology optimizations to all tasks
      sched_ext: idle: Explicitly pass allowed cpumask to scx_select_cpu_dfl()
      sched_ext: idle: Accept an arbitrary cpumask in scx_select_cpu_dfl()
      sched_ext: idle: Introduce scx_bpf_select_cpu_and()
      selftests/sched_ext: Add test for scx_bpf_select_cpu_and()
      sched_ext: idle: Deprecate scx_bpf_select_cpu_dfl()

 Documentation/scheduler/sched-ext.rst              |  11 +-
 kernel/sched/ext.c                                 |  13 +-
 kernel/sched/ext_idle.c                            | 265 +++++++++++++++------
 kernel/sched/ext_idle.h                            |   3 +-
 tools/sched_ext/include/scx/common.bpf.h           |   5 +-
 tools/sched_ext/include/scx/compat.bpf.h           |  37 +++
 tools/sched_ext/scx_flatcg.bpf.c                   |  12 +-
 tools/sched_ext/scx_simple.bpf.c                   |   9 +-
 tools/testing/selftests/sched_ext/Makefile         |   1 +
 .../testing/selftests/sched_ext/allowed_cpus.bpf.c |  91 +++++++
 tools/testing/selftests/sched_ext/allowed_cpus.c   |  57 +++++
 .../selftests/sched_ext/enq_select_cpu_fails.bpf.c |  12 +-
 .../selftests/sched_ext/enq_select_cpu_fails.c     |   2 +-
 tools/testing/selftests/sched_ext/exit.bpf.c       |   6 +-
 .../sched_ext/select_cpu_dfl_nodispatch.bpf.c      |  13 +-
 .../sched_ext/select_cpu_dfl_nodispatch.c          |   2 +-
 16 files changed, 424 insertions(+), 115 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/allowed_cpus.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/allowed_cpus.c